The present invention relates to an active matrix-type liquid crystal display having an MIM (Metal-Insulator-Metal) device as a switching device and a light shielding layer disposed in a non-display region on at least one of the substrates, which can be used suitably as a flat-panel display for office automation equipment, television sets, etc.
Most liquid crystal displays at present have now been changed from a simple matrix-type panel to an active matrix-type panel, because there is a demand for liquid crystal panels of large area in office automation terminal equipment, liquid crystal television sets, etc. In the active matrix-type, a switch is disposed to each picture element in order to hold a voltage.
MIM devices have often been used as one of the switches, because the MIM device shows an excellent non-linear current-voltage characteristic for switching. MIM devices known so far include one in which a lower metal electrode of Ta, Al or Ti is disposed on an insulation substrate such as a glass plate, an insulator film of SiO.sub.x, SiN.sub.x or an oxide of the metals mentioned above, disposed above the lower electrode and an upper metal electrode of Al or Cr disposed above the insulator film.
However, the MIM device using a metal oxide as an insulator (insulator layer) described in Japanese Patent Application Laid-Open (KOKAI) Nos. 57-196589 (1982), 61-232689 (1986) and 62-62333 (1987), has drawbacks as described below. Since the insulator layer is formed by anodization or thermal oxidation of the lower electrode, production steps are complicated and require high temperature heat-treatment necessary in anodization for sure removal of impurity, etc.). In addition, since the film controllability (homogenity and reproducibility of film quality and thickness) is poor, the substrate is limited to heat-resistant material, and the insulator layer should be made of a metal oxide of constant physical property. As a result, the material and the characteristic of the device can not be changed, thereby restricting the degrees of freedom in view of the design. This means that it is impossible to design and manufacture a device capable of sufficiently satisfying the specifications demanded for a liquid crystal display incorporated with the MIM devices.
Further, if the film controllability is poor, the current (I)-voltage (V) characteristic, and in particular, the symmetry of the I-V characteristic (current ratio I.sub.- /I.sub.30, between positive bias and negative bias) varies greatly.
In addition, in the case of using an MIM device for a liquid crystal display (LCD), since the ratio of liquid crystal capacitance/MIM device capacitance of not less than 10 is generally required, it is desirable that the capacity of the MIM device is smaller. However, since the dielectric constant of a metal oxide film is high, the device capacitance is also high. Accordingly, it is necessary for precision fabrication to reduce the device capacitance by reducing the device area. In this case, the insulator layer suffers from mechanical damage upon sealing the liquid crystal material (in the rubbing step, etc.). Also, the production yield is lowered, being coupled with the requirement in precision fabrication.
In an MIM device using SiO.sub.x or SiN.sub.x for the insulator layer (refer to Japanese Patent Application Laid-Open (KOKAI) No. 61-275819), the insulator layer is formed by a vapor phase method such as plasma CVD or sputtering. However, since a substrate temperature of about 300.degree. C. is usually necessary, it is impossible to use inexpensive substrates. In addition, upon preparing a large area device, the thickness and quality of the insulator layer may tend to be uneven owing to the temperature distribution of the substrate. Further, since the insulator layer is often formed in the vapor phase, dusts are generated in a great amount, resulting in a lot of pinholes in the insulator layer, thereby reducing the production yield of the device. Further, large stresses are caused in the insulator layer, bringing about peeling, and as a result, the device yield is lowered.
In addition, in a conventional liquid crystal display, switching devices such as the MIM device and lead electrodes are disposed in regions other than picture element electrodes. Accordingly, even if an opaque material such as a thin metal film is used for the electrode material, inter-electrode spaces, etc. are often present and as a result, so-called non-modulation light which is not effective for the operation of liquid crystals transmitted through the region. The non-modulation light means that leakage light is always present in the liquid crystal display, which reduces the contrast.
Further, a method of disposing a light-shielding layer on a substrate opposing the substrate of a switching device using TFT is known (Japanese Patent Application Laid-Open (KOKAI) No. 61-38931). A method of coating with a light-shielding insulator film on a region other than the picture element electrode on a substrate disposed with a non-linear device (mainly TFT) is also known. However, a liquid crystal display of low cost and with no picture-quality deterioration can not be obtained at present by merely employing these methods. Accordingly, it is an object of the present invention to provide a liquid crystal display of low cost with no picture quality deterioration by using a hard carbon film as a MIM insulating layer, which can also be used as a light blocking layer.
As a result of the present inventors' earnest studies for overcoming the foregoing drawbacks, it has been found that in a liquid crystal display comprising liquid crystal material sandwiched between two substrates, by (i) connecting each of a plurality of picture element electrodes disposed on at least one of the substrates with a lead electrode by means of at least one MIM (metal-insulator-metal) device; (ii) disposing a light-shielding layer on a region other than a display region of at least one of the substrates; and (iii) using a hard carbon film as the insulator of the MIM device, a liquid crystal display is obtainable at a relatively low temperature and by a simple method. The present liquid crystal display has highly reliable switching device having an insulator layer (hard carbon film) of low dielectric constant excellent in film controllability and mechanical strength. The present liquid crystal display is also capable of obtaining a high contrast display, with no leakage of light. The present invention has been accomplished based on the above-mentioned finding.